As typified by a liquid crystal display device, a thin film transistor formed over a flat board such as a glass substrate, is formed of amorphous silicon or polycrystalline silicon. A thin film transistor formed using amorphous silicon can cope with the increase of the glass substrate size even though electric field effect mobility is low. On the other hand, since a crystallization process such as laser annealing is needed, a thin film transistor formed using polycrystalline silicon does not always cope with the increase of the glass substrate size even though electric field effect mobility is high.
Accordingly, a technique of manufacturing a thin film transistor by using an oxide semiconductor and applying the thin film transistor to electronic devices and light devices is attracting attraction. For example, a technique of manufacturing a thin film transistor by using zinc oxide or In—Ga—Zn—O-based oxide semiconductor as the oxide semiconductor, and using the thin film transistor for a switching element of an image display device and the like is disclosed in Patent document 1 and Patent document 2.